The new ensurepip module (defined in PEP 453) provides a standard
cross-platform mechanism to bootstrap the pip installer into Python
installations and virtual environments. The version of pip included
with Python 3.4.0 is pip 1.5.4, and future 3.4.x maintenance releases
will update the bundled version to the latest version of pip that is
available at the time of creating the release candidate.

By default, the commands pipX and pipX.Y will be installed on all
platforms (where X.Y stands for the version of the Python installation),
along with the pip Python package and its dependencies. On Windows and
in virtual environments on all platforms, the unversioned pip command
will also be installed. On other platforms, the system wide unversioned
pip command typically refers to the separately installed Python 2
version.

The pyvenv command line utility and the venv
module make use of the ensurepip module to make pip readily
available in virtual environments. When using the command line utility,
pip is installed by default, while when using the venv module
API installation of pip must be requested explicitly.

For CPython source builds on POSIX systems,
the makeinstall and makealtinstall commands bootstrap pip by
default. This behaviour can be controlled through configure options, and
overridden through Makefile options.

On Windows and Mac OS X, the CPython installers now default to installing
pip along with CPython itself (users may opt out of installing it
during the installation process). Window users will need to opt in to the
automatic PATH modifications to have pip available from the command
line by default, otherwise it can still be accessed through the Python
launcher for Windows as py-mpip.

As discussed in the PEP, platform packagers may choose not to install
these commands by default, as long as, when invoked, they provide clear and
simple directions on how to install them on that platform (usually using
the system package manager).

Note

To avoid conflicts between parallel Python 2 and Python 3 installations,
only the versioned pip3 and pip3.4 commands are bootstrapped by
default when ensurepip is invoked directly - the --default-pip
option is needed to also request the unversioned pip command.
pyvenv and the Windows installer ensure that the unqualified pip
command is made available in those environments, and pip can always be
invoked via the -m switch rather than directly to avoid ambiguity on
systems with multiple Python installations.

PEP 446 makes newly created file descriptors non-inheritable. In general, this is the behavior an application will
want: when launching a new process, having currently open files also
open in the new process can lead to all sorts of hard to find bugs,
and potentially to security issues.

However, there are occasions when inheritance is desired. To support
these cases, the following new functions and methods are available:

Since it was first introduced, the codecs module has always been
intended to operate as a type-neutral dynamic encoding and decoding
system. However, its close coupling with the Python text model, especially
the type restricted convenience methods on the builtin str,
bytes and bytearray types, has historically obscured that
fact.

As a key step in clarifying the situation, the codecs.encode() and
codecs.decode() convenience functions are now properly documented in
Python 2.7, 3.3 and 3.4. These functions have existed in the codecs
module (and have been covered by the regression test suite) since Python 2.4,
but were previously only discoverable through runtime introspection.

Unlike the convenience methods on str, bytes and
bytearray, the codecs convenience functions support arbitrary
codecs in both Python 2 and Python 3, rather than being limited to Unicode text
encodings (in Python 3) or basestring <-> basestring conversions (in
Python 2).

In Python 3.4, the interpreter is able to identify the known non-text
encodings provided in the standard library and direct users towards these
general purpose convenience functions when appropriate:

>>> b"abcdef".decode("hex")Traceback (most recent call last):
File "<stdin>", line 1, in <module>LookupError: 'hex' is not a text encoding; use codecs.decode() to handle arbitrary codecs>>> "hello".encode("rot13")Traceback (most recent call last):
File "<stdin>", line 1, in <module>LookupError: 'rot13' is not a text encoding; use codecs.encode() to handle arbitrary codecs>>> open("foo.txt",encoding="hex")Traceback (most recent call last):
File "<stdin>", line 1, in <module>LookupError: 'hex' is not a text encoding; use codecs.open() to handle arbitrary codecs

In a related change, whenever it is feasible without breaking backwards
compatibility, exceptions raised during encoding and decoding operations
are wrapped in a chained exception of the same type that mentions the
name of the codec responsible for producing the error:

Finally, as the examples above show, these improvements have permitted
the restoration of the convenience aliases for the non-Unicode codecs that
were themselves restored in Python 3.2. This means that encoding binary data
to and from its hexadecimal representation (for example) can now be written
as:

PEP 451 provides an encapsulation of the information about a module that the
import machinery will use to load it (that is, a module specification). This
helps simplify both the import implementation and several import-related APIs.
The change is also a stepping stone for several future import-related
improvements.

The public-facing changes from the PEP are entirely backward-compatible.
Furthermore, they should be transparent to everyone but importer authors. Key
finder and loader methods have been deprecated, but they will continue working.
New importers should use the new methods described in the PEP. Existing
importers should be updated to implement the new methods. See the
Deprecated section for a list of methods that should be replaced and
their replacements.

min() and max() now accept a default keyword-only argument that
can be used to specify the value they return if the iterable they are
evaluating has no elements. (Contributed by Julian Berman in
bpo-18111.)

Module __file__ attributes (and related values) should now always
contain absolute paths by default, with the sole exception of
__main__.__file__ when a script has been executed directly using
a relative path. (Contributed by Brett Cannon in bpo-18416.)

All the UTF-* codecs (except UTF-7) now reject surrogates during both
encoding and decoding unless the surrogatepass error handler is used,
with the exception of the UTF-16 decoder (which accepts valid surrogate pairs)
and the UTF-16 encoder (which produces them while encoding non-BMP characters).
(Contributed by Victor Stinner, Kang-Hao (Kenny) Lu and Serhiy Storchaka in
bpo-12892.)

New German EBCDIC codeccp273. (Contributed
by Michael Bierenfeld and Andrew Kuchling in bpo-1097797.)

New Ukrainian codeccp1125. (Contributed by
Serhiy Storchaka in bpo-19668.)

bytes.join() and bytearray.join() now accept arbitrary
buffer objects as arguments. (Contributed by Antoine Pitrou in
bpo-15958.)

The int constructor now accepts any object that has an __index__
method for its base argument. (Contributed by Mark Dickinson in
bpo-16772.)

Frame objects now have a clear() method that clears all
references to local variables from the frame. (Contributed by Antoine Pitrou
in bpo-17934.)

The new asyncio module (defined in PEP 3156) provides a standard
pluggable event loop model for Python, providing solid asynchronous IO
support in the standard library, and making it easier for other event loop
implementations to interoperate with the standard library and each other.

The new ensurepip module is the primary infrastructure for the
PEP 453 implementation. In the normal course of events end users will not
need to interact with this module, but it can be used to manually bootstrap
pip if the automated bootstrapping into an installation or virtual
environment was declined.

ensurepip includes a bundled copy of pip, up-to-date as of the first
release candidate of the release of CPython with which it ships (this applies
to both maintenance releases and feature releases). ensurepip does not
access the internet. If the installation has Internet access, after
ensurepip is run the bundled pip can be used to upgrade pip to a
more recent release than the bundled one. (Note that such an upgraded version
of pip is considered to be a separately installed package and will not be
removed if Python is uninstalled.)

The module is named ensurepip because if called when pip is already
installed, it does nothing. It also has an --upgrade option that will
cause it to install the bundled copy of pip if the existing installed
version of pip is older than the bundled copy.

The new pathlib module offers classes representing filesystem paths
with semantics appropriate for different operating systems. Path classes are
divided between pure paths, which provide purely computational operations
without I/O, and concrete paths, which inherit from pure paths but also
provide I/O operations.

The new statistics module (defined in PEP 450) offers some core
statistics functionality directly in the standard library. This module
supports calculation of the mean, median, mode, variance and standard
deviation of a data series.

New function abc.get_cache_token() can be used to know when to invalidate
caches that are affected by changes in the object graph. (Contributed
by Łukasz Langa in bpo-16832.)

New class ABC has ABCMeta as its meta class.
Using ABC as a base class has essentially the same effect as specifying
metaclass=abc.ABCMeta, but is simpler to type and easier to read.
(Contributed by Bruno Dupuis in bpo-16049.)

The getparams() method now returns a namedtuple rather than a
plain tuple. (Contributed by Claudiu Popa in bpo-17818.)

aifc.open() now supports the context management protocol: when used in a
with block, the close() method of the returned
object will be called automatically at the end of the block. (Contributed by
Serhiy Storchacha in bpo-16486.)

New functions a85encode(), a85decode(),
b85encode(), and b85decode() provide the ability to
encode and decode binary data from and to Ascii85 and the git/mercurial
Base85 formats, respectively. The a85 functions have options that can
be used to make them compatible with the variants of the Ascii85 encoding,
including the Adobe variant. (Contributed by Martin Morrison, the Mercurial
project, Serhiy Storchaka, and Antoine Pitrou in bpo-17618.)

The ChainMap.new_child() method now accepts an m argument specifying
the child map to add to the chain. This allows an existing mapping and/or a
custom mapping type to be used for the child. (Contributed by Vinay Sajip in
bpo-16613.)

The number of digits in the coefficients for the RGB — YIQ conversions have
been expanded so that they match the FCC NTSC versions. The change in
results should be less than 1% and may better match results found elsewhere.
(Contributed by Brian Landers and Serhiy Storchaka in bpo-14323.)

The new contextlib.suppress context manager helps to clarify the
intent of code that deliberately suppresses exceptions from a single
statement. (Contributed by Raymond Hettinger in bpo-15806 and
Zero Piraeus in bpo-19266.)

The new contextlib.redirect_stdout() context manager makes it easier
for utility scripts to handle inflexible APIs that write their output to
sys.stdout and don’t provide any options to redirect it. Using the
context manager, the sys.stdout output can be redirected to any
other stream or, in conjunction with io.StringIO, to a string.
The latter can be especially useful, for example, to capture output
from a function that was written to implement a command line interface.
It is recommended only for utility scripts because it affects the
global state of sys.stdout. (Contributed by Raymond Hettinger
in bpo-15805.)

The contextlib documentation has also been updated to include a
discussion of the
differences between single use, reusable and reentrant context managers.

dbm.open() objects now support the context management protocol. When
used in a with statement, the close method of the database
object will be called automatically at the end of the block. (Contributed by
Claudiu Popa and Nick Coghlan in bpo-19282.)

The dis module is now built around an Instruction class
that provides object oriented access to the details of each individual bytecode
operation.

A new method, get_instructions(), provides an iterator that emits
the Instruction stream for a given piece of Python code. Thus it is now
possible to write a program that inspects and manipulates a bytecode
object in ways different from those provided by the dis module
itself. For example:

The various display tools in the dis module have been rewritten to use
these new components.

In addition, a new application-friendly class Bytecode provides
an object-oriented API for inspecting bytecode in both in human-readable form
and for iterating over instructions. The Bytecode constructor
takes the same arguments that get_instruction() does (plus an
optional current_offset), and the resulting object can be iterated to produce
Instruction objects. But it also has a dis
method, equivalent to calling dis on the constructor argument, but
returned as a multi-line string:

A new option flag, FAIL_FAST, halts
test running as soon as the first failure is detected. (Contributed by R.
David Murray and Daniel Urban in bpo-16522.)

The doctest command line interface now uses argparse, and has two
new options, -o and -f. -o allows doctest options to be specified on the command line, and -f is a
shorthand for -oFAIL_FAST (to parallel the similar option supported by the
unittest CLI). (Contributed by R. David Murray in bpo-11390.)

doctest will now find doctests in extension module __doc__ strings.
(Contributed by Zachary Ware in bpo-3158.)

as_string() now accepts a policy argument to
override the default policy of the message when generating a string
representation of it. This means that as_string can now be used in more
circumstances, instead of having to create and use a generator in
order to pass formatting parameters to its flatten method. (Contributed by
R. David Murray in bpo-18600.)

New method as_bytes() added to produce a bytes
representation of the message in a fashion similar to how as_string
produces a string representation. It does not accept the maxheaderlen
argument, but does accept the unixfrom and policy arguments. The
Message__bytes__() method
calls it, meaning that bytes(mymsg) will now produce the intuitive
result: a bytes object containing the fully formatted message. (Contributed
by R. David Murray in bpo-18600.)

The Message.set_param() message now accepts a replace keyword argument.
When specified, the associated header will be updated without changing
its location in the list of headers. For backward compatibility, the default
is False. (Contributed by R. David Murray in bpo-18891.)

A pair of new subclasses of Message have been added
(EmailMessage and MIMEPart), along with a new sub-module,
contentmanager and a new policy attribute
content_manager. All documentation is
currently in the new module, which is being added as part of email’s new
provisional API. These classes provide a number of new methods that
make extracting content from and inserting content into email messages much
easier. For details, see the contentmanager documentation and
the email: Examples. These API additions complete the
bulk of the work that was planned as part of the email6 project. The currently
provisional API is scheduled to become final in Python 3.5 (possibly with a few
minor additions in the area of error handling). (Contributed by R. David
Murray in bpo-18891.)

A new clear_cache() function provides the ability to clear the
filecmp comparison cache, which uses os.stat() information to
determine if the file has changed since the last compare. This can be used,
for example, if the file might have been changed and re-checked in less time
than the resolution of a particular filesystem’s file modification time field.
(Contributed by Mark Levitt in bpo-18149.)

New module attribute DEFAULT_IGNORES provides the list of
directories that are used as the default value for the ignore parameter of
the dircmp() function. (Contributed by Eli Bendersky in
bpo-15442.)

The new partialmethod() descriptor brings partial argument
application to descriptors, just as partial() provides
for normal callables. The new descriptor also makes it easier to get
arbitrary callables (including partial() instances)
to behave like normal instance methods when included in a class definition.
(Contributed by Alon Horev and Nick Coghlan in bpo-4331.)

The new singledispatch() decorator brings support for
single-dispatch generic functions to the Python standard library. Where
object oriented programming focuses on grouping multiple operations on a
common set of data into a class, a generic function focuses on grouping
multiple implementations of an operation that allows it to work with
different kinds of data.

A pure-python version of the partial() function is now in the
stdlib; in CPython it is overridden by the C accelerated version, but it is
available for other implementations to use. (Contributed by Brian Thorne in
bpo-12428.)

A new function escape() provides a way to escape special characters
in a filename so that they do not become part of the globbing expansion but are
instead matched literally. (Contributed by Serhiy Storchaka in bpo-8402.)

The name attribute of hashlib hash objects is now
a formally supported interface. It has always existed in CPython’s
hashlib (although it did not return lower case names for all supported
hashes), but it was not a public interface and so some other Python
implementations have not previously supported it. (Contributed by Jason R.
Coombs in bpo-18532.)

hmac now accepts bytearray as well as bytes for the key
argument to the new() function, and the msg parameter to both the
new() function and the update() method now
accepts any type supported by the hashlib module. (Contributed
by Jonas Borgström in bpo-18240.)

The digestmod argument to the hmac.new() function may now be any hash
digest name recognized by hashlib. In addition, the current behavior in
which the value of digestmod defaults to MD5 is deprecated: in a
future version of Python there will be no default value. (Contributed by
Christian Heimes in bpo-17276.)

New function unescape() function converts HTML5 character references to
the corresponding Unicode characters. (Contributed by Ezio Melotti in
bpo-2927.)

HTMLParser accepts a new keyword argument
convert_charrefs that, when True, automatically converts all character
references. For backward-compatibility, its value defaults to False, but
it will change to True in a future version of Python, so you are invited to
set it explicitly and update your code to use this new feature. (Contributed
by Ezio Melotti in bpo-13633.)

The strict argument of HTMLParser is now deprecated.
(Contributed by Ezio Melotti in bpo-15114.)

send_error() now accepts an
optional additional explain parameter which can be used to provide an
extended error description, overriding the hardcoded default if there is one.
This extended error description will be formatted using the
error_message_format attribute and sent as the body
of the error response. (Contributed by Karl Cow in bpo-12921.)

Since idlelib implements the IDLE shell and editor and is not intended for
import by other programs, it gets improvements with every release. See
Lib/idlelib/NEWS.txt for a cumulative list of changes since 3.3.0,
as well as changes made in future 3.4.x releases. This file is also available
from the IDLE Help ‣ About IDLE dialog.

The InspectLoader ABC defines a new method,
source_to_code() that accepts source
data and a path and returns a code object. The default implementation
is equivalent to compile(data,path,'exec',dont_inherit=True).
(Contributed by Eric Snow and Brett Cannon in bpo-15627.)

InspectLoader also now has a default implementation
for the get_code() method. However,
it will normally be desirable to override the default implementation
for performance reasons. (Contributed by Brett Cannon in bpo-18072.)

The inspect module now offers a basic command line interface to quickly display source code and other
information for modules, classes and functions. (Contributed by Claudiu Popa
and Nick Coghlan in bpo-18626.)

unwrap() makes it easy to unravel wrapper function chains
created by functools.wraps() (and any other API that sets the
__wrapped__ attribute on a wrapper function). (Contributed by
Daniel Urban, Aaron Iles and Nick Coghlan in bpo-13266.)

As part of the implementation of the new enum module, the
inspect module now has substantially better support for custom
__dir__ methods and dynamic class attributes provided through
metaclasses. (Contributed by Ethan Furman in bpo-18929 and
bpo-19030.)

getfullargspec() and getargspec()
now use the signature() API. This allows them to
support a much broader range of callables, including those with
__signature__ attributes, those with metadata provided by argument
clinic, functools.partial() objects and more. Note that, unlike
signature(), these functions still ignore __wrapped__
attributes, and report the already bound first argument for bound methods,
so it is still necessary to update your code to use
signature() directly if those features are desired.
(Contributed by Yury Selivanov in bpo-17481.)

signature() now supports duck types of CPython functions,
which adds support for functions compiled with Cython. (Contributed
by Stefan Behnel and Yury Selivanov in bpo-17159.)

ipaddress was added to the standard library in Python 3.3 as a
provisional API. With the release of Python 3.4, this qualification
has been removed: ipaddress is now considered a stable API, covered
by the normal standard library requirements to maintain backwards
compatibility.

A new is_global property is True if
an address is globally routeable. (Contributed by Peter Moody in
bpo-17400.)

The TimedRotatingFileHandler has a new atTime
parameter that can be used to specify the time of day when rollover should
happen. (Contributed by Ronald Oussoren in bpo-9556.)

SocketHandler and
DatagramHandler now support Unix domain sockets (by
setting port to None). (Contributed by Vinay Sajip in commit
ce46195b56a9.)

fileConfig() now accepts a
configparser.RawConfigParser subclass instance for the fname
parameter. This facilitates using a configuration file when logging
configuration is just a part of the overall application configuration, or where
the application modifies the configuration before passing it to
fileConfig(). (Contributed by Vinay Sajip in
bpo-16110.)

Logging configuration data received from a socket via the
logging.config.listen() function can now be validated before being
processed by supplying a verification function as the argument to the new
verify keyword argument. (Contributed by Vinay Sajip in bpo-15452.)

The default marshal version has been bumped to 3. The code implementing
the new version restores the Python2 behavior of recording only one copy of
interned strings and preserving the interning on deserialization, and extends
this “one copy” ability to any object type (including handling recursive
references). This reduces both the size of .pyc files and the amount of
memory a module occupies in memory when it is loaded from a .pyc (or
.pyo) file. (Contributed by Kristján Valur Jónsson in bpo-16475,
with additional speedups by Antoine Pitrou in bpo-19219.)

On Unix two new start methods,
spawn and forkserver, have been added for starting processes using
multiprocessing. These make the mixing of processes with threads more
robust, and the spawn method matches the semantics that multiprocessing has
always used on Windows. New function
get_all_start_methods() reports all start methods
available on the platform, get_start_method() reports
the current start method, and set_start_method() sets
the start method. (Contributed by Richard Oudkerk in bpo-8713.)

multiprocessing also now has the concept of a context, which
determines how child processes are created. New function
get_context() returns a context that uses a specified
start method. It has the same API as the multiprocessing module itself,
so you can use it to create Pools and other
objects that will operate within that context. This allows a framework and an
application or different parts of the same application to use multiprocessing
without interfering with each other. (Contributed by Richard Oudkerk in
bpo-18999.)

Except when using the old fork start method, child processes no longer
inherit unneeded handles/file descriptors from their parents (part of
bpo-8713).

multiprocessing now relies on runpy (which implements the
-m switch) to initialise __main__ appropriately in child processes
when using the spawn or forkserver start methods. This resolves some
edge cases where combining multiprocessing, the -m command line switch,
and explicit relative imports could cause obscure failures in child
processes. (Contributed by Nick Coghlan in bpo-19946.)

New function length_hint() provides an implementation of the
specification for how the __length_hint__() special method should
be used, as part of the PEP 424 formal specification of this language
feature. (Contributed by Armin Ronacher in bpo-16148.)

There is now a pure-python version of the operator module available for
reference and for use by alternate implementations of Python. (Contributed by
Zachary Ware in bpo-16694.)

New function cpu_count() reports the number of CPUs available on the
platform on which Python is running (or None if the count can’t be
determined). The multiprocessing.cpu_count() function is now implemented
in terms of this function). (Contributed by Trent Nelson, Yogesh Chaudhari,
Victor Stinner, and Charles-François Natali in bpo-17914.)

os.open() supports two new flags on platforms that provide them,
O_PATH (un-opened file descriptor), and O_TMPFILE
(unnamed temporary file; as of 3.4.0 release available only on Linux systems
with a kernel version of 3.11 or newer that have uapi headers). (Contributed
by Christian Heimes in bpo-18673 and Benjamin Peterson, respectively.)

pdb has been enhanced to handle generators, yield, and
yieldfrom in a more useful fashion. This is especially helpful when
debugging asyncio based programs. (Contributed by Andrew Svetlov and
Xavier de Gaye in bpo-16596.)

The print command has been removed from pdb, restoring access to the
Python print() function from the pdb command line. Python2’s pdb did
not have a print command; instead, entering print executed the
print statement. In Python3 print was mistakenly made an alias for the
pdb p command. p, however, prints the repr of its argument,
not the str like the Python2 print command did. Worse, the Python3
pdbprint command shadowed the Python3 print function, making it
inaccessible at the pdb prompt. (Contributed by Connor Osborn in
bpo-18764.)

pickle now supports (but does not use by default) a new pickle protocol,
protocol 4. This new protocol addresses a number of issues that were present
in previous protocols, such as the serialization of nested classes, very large
strings and containers, and classes whose __new__() method takes
keyword-only arguments. It also provides some efficiency improvements.

plistlib now has an API that is similar to the standard pattern for
stdlib serialization protocols, with new load(),
dump(), loads(), and dumps()
functions. (The older API is now deprecated.) In addition to the already
supported XML plist format (FMT_XML), it also now supports
the binary plist format (FMT_BINARY). (Contributed by Ronald
Oussoren and others in bpo-14455.)

Two new methods have been added to poplib: capa(),
which returns the list of capabilities advertised by the POP server, and
stls(), which switches a clear-text POP3 session into an
encrypted POP3 session if the POP server supports it. (Contributed by Lorenzo
Catucci in bpo-4473.)

The pprint module’s PrettyPrinter class and its
pformat(), and pprint() functions have a new
option, compact, that controls how the output is formatted. Currently
setting compact to True means that sequences will be printed with as many
sequence elements as will fit within width on each (indented) line.
(Contributed by Serhiy Storchaka in bpo-19132.)

Long strings are now wrapped using Python’s normal line continuation
syntax. (Contributed by Antoine Pitrou in bpo-17150.)

The pydoc module is now based directly on the inspect.signature()
introspection API, allowing it to provide signature information for a wider
variety of callable objects. This change also means that __wrapped__
attributes are now taken into account when displaying help information.
(Contributed by Larry Hastings in bpo-19674.)

The pydoc module no longer displays the self parameter for
already bound methods. Instead, it aims to always display the exact current
signature of the supplied callable. (Contributed by Larry Hastings in
bpo-20710.)

In addition to the changes that have been made to pydoc directly,
its handling of custom __dir__ methods and various descriptor
behaviours has also been improved substantially by the underlying changes in
the inspect module.

As the help() builtin is based on pydoc, the above changes also
affect the behaviour of help().

New fullmatch() function and regex.fullmatch() method anchor
the pattern at both ends of the string to match. This provides a way to be
explicit about the goal of the match, which avoids a class of subtle bugs where
$ characters get lost during code changes or the addition of alternatives
to an existing regular expression. (Contributed by Matthew Barnett in
bpo-16203.)

The repr of regex objects now includes the pattern
and the flags; the repr of match objects now
includes the start, end, and the part of the string that matched. (Contributed
by Hugo Lopes Tavares and Serhiy Storchaka in bpo-13592 and
bpo-17087.)

New prlimit() function, available on Linux platforms with a
kernel version of 2.6.36 or later and glibc of 2.13 or later, provides the
ability to query or set the resource limits for processes other than the one
making the call. (Contributed by Christian Heimes in bpo-16595.)

epoll objects now support the context management protocol.
When used in a with statement, the close()
method will be called automatically at the end of the block. (Contributed
by Serhiy Storchaka in bpo-16488.)

copyfile() now raises a specific Error subclass,
SameFileError, when the source and destination are the same
file, which allows an application to take appropriate action on this specific
error. (Contributed by Atsuo Ishimoto and Hynek Schlawack in
bpo-1492704.)

The SMTPServer and SMTPChannel classes now
accept a map keyword argument which, if specified, is passed in to
asynchat.async_chat as its map argument. This allows an application
to avoid affecting the global socket map. (Contributed by Vinay Sajip in
bpo-11959.)

SMTPException is now a subclass of OSError, which allows
both socket level errors and SMTP protocol level errors to be caught in one
try/except statement by code that only cares whether or not an error occurred.
(Contributed by Ned Jackson Lovely in bpo-2118.)

PROTOCOL_TLSv1_1 and PROTOCOL_TLSv1_2 (TLSv1.1 and
TLSv1.2 support) have been added; support for these protocols is only available if
Python is linked with OpenSSL 1.0.1 or later. (Contributed by Michele Orrù and
Antoine Pitrou in bpo-16692.)

New function create_default_context() provides a standard way to
obtain an SSLContext whose settings are intended to be a
reasonable balance between compatibility and security. These settings are
more stringent than the defaults provided by the SSLContext
constructor, and may be adjusted in the future, without prior deprecation, if
best-practice security requirements change. The new recommended best
practice for using stdlib libraries that support SSL is to use
create_default_context() to obtain an SSLContext
object, modify it if needed, and then pass it as the context argument
of the appropriate stdlib API. (Contributed by Christian Heimes
in bpo-19689.)

SSLContext method load_verify_locations()
accepts a new optional argument cadata, which can be used to provide PEM or
DER encoded certificates directly via strings or bytes, respectively.
(Contributed by Christian Heimes in bpo-18138.)

SSLContext has a new method,
cert_store_stats(), that reports the number of loaded
X.509 certs, X.509CA certs, and certificate revocation lists
(crls), as well as a get_ca_certs() method that
returns a list of the loaded CA certificates. (Contributed by Christian
Heimes in bpo-18147.)

New SSLContext method load_default_certs()
loads a set of default “certificate authority” (CA) certificates from default
locations, which vary according to the platform. It can be used to load both
TLS web server authentication certificates
(purpose=SERVER_AUTH) for a client to use to verify a
server, and certificates for a server to use in verifying client certificates
(purpose=CLIENT_AUTH). (Contributed by Christian
Heimes in bpo-19292.)

Two new windows-only functions, enum_certificates() and
enum_crls() provide the ability to retrieve certificates,
certificate information, and CRLs from the Windows cert store. (Contributed
by Christian Heimes in bpo-17134.)

The stat module is now backed by a C implementation in _stat. A C
implementation is required as most of the values aren’t standardized and
are platform-dependent. (Contributed by Christian Heimes in bpo-11016.)

The getparams() method now returns a namedtuple rather than a
plain tuple. (Contributed by Claudiu Popa in bpo-18901.)

sunau.open() now supports the context management protocol: when used in a
with block, the close method of the returned object will be
called automatically at the end of the block. (Contributed by Serhiy Storchaka
in bpo-18878.)

AU_write.setsampwidth() now supports 24 bit samples, thus adding
support for writing 24 sample using the module. (Contributed by
Serhiy Storchaka in bpo-19261.)

New function sys.getallocatedblocks() returns the current number of
blocks allocated by the interpreter. (In CPython with the default
--with-pymalloc setting, this is allocations made through the
PyObject_Malloc() API.) This can be useful for tracking memory leaks,
especially if automated via a test suite. (Contributed by Antoine Pitrou
in bpo-13390.)

When the Python interpreter starts in interactive mode, it checks for an __interactivehook__ attribute
on the sys module. If the attribute exists, its value is called with no
arguments just before interactive mode is started. The check is made after the
PYTHONSTARTUP file is read, so it can be set there. The site
module sets it to a function that enables tab
completion and history saving (in ~/.python-history) if the platform
supports readline. If you do not want this (new) behavior, you can
override it in PYTHONSTARTUP, sitecustomize, or
usercustomize by deleting this attribute from sys (or setting it
to some other callable). (Contributed by Éric Araujo and Antoine Pitrou in
bpo-5845.)

The tarfile module now supports a simple Command-Line Interface when
called as a script directly or via -m. This can be used to create and
extract tarfile archives. (Contributed by Berker Peksag in bpo-13477.)

The TextWrapper class has two new attributes/constructor
arguments: max_lines, which limits the number of
lines in the output, and placeholder, which is a
string that will appear at the end of the output if it has been truncated
because of max_lines. Building on these capabilities, a new convenience
function shorten() collapses all of the whitespace in the input
to single spaces and produces a single line of a given width that ends with
the placeholder (by default, [...]). (Contributed by Antoine Pitrou and
Serhiy Storchaka in bpo-18585 and bpo-18725.)

The Thread object representing the main thread can be
obtained from the new main_thread() function. In normal
conditions this will be the thread from which the Python interpreter was
started. (Contributed by Andrew Svetlov in bpo-18882.)

A new traceback.clear_frames() function takes a traceback object
and clears the local variables in all of the frames it references,
reducing the amount of memory consumed. (Contributed by Andrew Kuchling in
bpo-1565525.)

A new DynamicClassAttribute() descriptor provides a way to define
an attribute that acts normally when looked up through an instance object, but
which is routed to the class__getattr__ when looked up through the
class. This allows one to have properties active on a class, and have virtual
attributes on the class with the same name (see Enum for an example).
(Contributed by Ethan Furman in bpo-19030.)

The http method that will be used by a Request class
can now be specified by setting a method
class attribute on the subclass. (Contributed by Jason R Coombs in
bpo-18978.)

Request objects are now reusable: if the
full_url or data
attributes are modified, all relevant internal properties are updated. This
means, for example, that it is now possible to use the same
Request object in more than one
OpenerDirector.open() call with different data arguments, or to
modify a Request’s url rather than recomputing it
from scratch. There is also a new
remove_header() method that can be used to remove
headers from a Request. (Contributed by Alexey
Kachayev in bpo-16464, Daniel Wozniak in bpo-17485, and Damien Brecht
and Senthil Kumaran in bpo-17272.)

HTTPError objects now have a
headers attribute that provides access to the
HTTP response headers associated with the error. (Contributed by
Berker Peksag in bpo-15701.)

The TestCase class has a new method,
subTest(), that produces a context manager whose
with block becomes a “sub-test”. This context manager allows a test
method to dynamically generate subtests by, say, calling the subTest
context manager inside a loop. A single test method can thereby produce an
indefinite number of separately-identified and separately-counted tests, all of
which will run even if one or more of them fail. For example:

will result in six subtests, each identified in the unittest verbose output
with a label consisting of the variable name i and a particular value for
that variable (i=0, i=1, etc). See Distinguishing test iterations using subtests for the full
version of this example. (Contributed by Antoine Pitrou in bpo-16997.)

unittest.main() now accepts an iterable of test names for
defaultTest, where previously it only accepted a single test name as a
string. (Contributed by Jyrki Pulliainen in bpo-15132.)

If SkipTest is raised during test discovery (that is, at the
module level in the test file), it is now reported as a skip instead of an
error. (Contributed by Zach Ware in bpo-16935.)

discover() now sorts the discovered files to provide
consistent test ordering. (Contributed by Martin Melin and Jeff Ramnani in
bpo-16709.)

TestSuite now drops references to tests as soon as the test
has been run, if the test is successful. On Python interpreters that do
garbage collection, this allows the tests to be garbage collected if nothing
else is holding a reference to the test. It is possible to override this
behavior by creating a TestSuite subclass that defines a
custom _removeTestAtIndex method. (Contributed by Tom Wardill, Matt
McClure, and Andrew Svetlov in bpo-11798.)

A new test assertion context-manager, assertLogs(),
will ensure that a given block of code emits a log message using the
logging module. By default the message can come from any logger and
have a priority of INFO or higher, but both the logger name and an
alternative minimum logging level may be specified. The object returned by the
context manager can be queried for the LogRecords and/or
formatted messages that were logged. (Contributed by Antoine Pitrou in
bpo-18937.)

Test discovery now works with namespace packages (Contributed by Claudiu Popa
in bpo-17457.)

unittest.mock objects now inspect their specification signatures when
matching calls, which means an argument can now be matched by either position
or name, instead of only by position. (Contributed by Antoine Pitrou in
bpo-17015.)

mock_open() objects now have readline and readlines
methods. (Contributed by Toshio Kuratomi in bpo-17467.)

venv now includes activation scripts for the csh and fish
shells. (Contributed by Andrew Svetlov in bpo-15417.)

EnvBuilder and the create() convenience function
take a new keyword argument with_pip, which defaults to False, that
controls whether or not EnvBuilder ensures that pip is
installed in the virtual environment. (Contributed by Nick Coghlan in
bpo-19552 as part of the PEP 453 implementation.)

New finalize class makes it possible to register a callback
to be invoked when an object is garbage collected, without needing to
carefully manage the lifecycle of the weak reference itself. (Contributed by
Richard Oudkerk in bpo-15528.)

The callback, if any, associated with a ref is now
exposed via the __callback__ attribute. (Contributed
by Mark Dickinson in bpo-17643.)

The writepy() method of the
PyZipFile class has a new filterfunc option that can be
used to control which directories and files are added to the archive. For
example, this could be used to exclude test files from the archive.
(Contributed by Christian Tismer in bpo-19274.)

The allowZip64 parameter to ZipFile and
PyZipfile is now True by default. (Contributed by
William Mallard in bpo-17201.)

PEP 442 removes the current limitations and quirks of object finalization
in CPython. With it, objects with __del__() methods, as well as
generators with finally clauses, can be finalized when they are
part of a reference cycle.

As part of this change, module globals are no longer forcibly set to
None during interpreter shutdown in most cases, instead relying
on the normal operation of the cyclic garbage collector. This avoids a
whole class of interpreter-shutdown-time errors, usually involving
__del__ methods, that have plagued Python since the cyclic GC
was first introduced.

PEP 456 follows up on earlier security fix work done on Python’s hash
algorithm to address certain DOS attacks to which public facing APIs backed by
dictionary lookups may be subject. (See bpo-14621 for the start of the
current round of improvements.) The PEP unifies CPython’s hash code to make it
easier for a packager to substitute a different hash algorithm, and switches
Python’s default implementation to a SipHash implementation on platforms that
have a 64 bit data type. Any performance differences in comparison with the
older FNV algorithm are trivial.

The PEP adds additional fields to the sys.hash_info struct sequence to
describe the hash algorithm in use by the currently executing binary. Otherwise,
the PEP does not alter any existing CPython APIs.

“Argument Clinic” (PEP 436) is now part of the CPython build process
and can be used to simplify the process of defining and maintaining
accurate signatures for builtins and standard library extension modules
implemented in C.

Some standard library extension modules have been converted to use Argument
Clinic in Python 3.4, and pydoc and inspect have been updated
accordingly.

It is expected that signature metadata for programmatic introspection will
be added to additional callables implemented in C as part of Python 3.4
maintenance releases.

Note

The Argument Clinic PEP is not fully up to date with the state of the
implementation. This has been deemed acceptable by the release manager
and core development team in this case, as Argument Clinic will not
be made available as a public API for third party use in Python 3.4.

The new PyType_GetSlot() function has been added to the stable ABI,
allowing retrieval of function pointers from named type slots when using
the limited API. (Contributed by Martin von Löwis in bpo-17162.)

The new Py_SetStandardStreamEncoding() pre-initialization API
allows applications embedding the CPython interpreter to reliably force
a particular encoding and error handler for the standard streams.
(Contributed by Bastien Montagne and Nick Coghlan in bpo-16129.)

Most Python C APIs that don’t mutate string arguments are now correctly
marked as accepting constchar* rather than char*. (Contributed
by Serhiy Storchaka in bpo-1772673.)

A new shell version of python-config can be used even when a python
interpreter is not available (for example, in cross compilation scenarios).

The CPython source can now be compiled using the address sanity checking
features of recent versions of GCC and clang: the false alarms in the small
object allocator have been silenced. (Contributed by Dhiru Kholia in
bpo-18596.)

The python command has a new option, -I, which causes it to run in “isolated mode”,
which means that sys.path contains neither the script’s directory nor
the user’s site-packages directory, and all PYTHON* environment
variables are ignored (it implies both -s and -E). Other
restrictions may also be applied in the future, with the goal being to
isolate the execution of a script from the user’s environment. This is
appropriate, for example, when Python is used to run a system script. On
most POSIX systems it can and should be used in the #! line of system
scripts. (Contributed by Christian Heimes in bpo-16499.)

Tab-completion is now enabled by default in the interactive interpreter
on systems that support readline. History is also enabled by default,
and is written to (and read from) the file ~/.python-history.
(Contributed by Antoine Pitrou and Éric Araujo in bpo-5845.)

Invoking the Python interpreter with --version now outputs the version to
standard output instead of standard error (bpo-18338). Similar changes
were made to argparse (bpo-18920) and other modules that have
script-like invocation capabilities (bpo-18922).

The CPython Windows installer now adds .py to the PATHEXT
variable when extensions are registered, allowing users to run a python
script at the windows command prompt by just typing its name without the
.py extension. (Contributed by Paul Moore in bpo-18569.)

A new make target coverage-report
will build python, run the test suite, and generate an HTML coverage report
for the C codebase using gcov and lcov.

The stat module is now implemented in C, which means it gets the
values for its constants from the C header files, instead of having the
values hard-coded in the python module as was previously the case.

Loading multiple python modules from a single OS module (.so, .dll)
now works correctly (previously it silently returned the first python
module in the file). (Contributed by Václav Šmilauer in bpo-16421.)

A new opcode, LOAD_CLASSDEREF, has been added to fix a bug in the
loading of free variables in class bodies that could be triggered by certain
uses of __prepare__. (Contributed by Benjamin Peterson in
bpo-17853.)

A number of MemoryError-related crashes were identified and fixed by Victor
Stinner using his PEP 445-based pyfailmalloc tool (bpo-18408,
bpo-18520).

The pyvenv command now accepts a --copies option
to use copies rather than symlinks even on systems where symlinks are the
default. (Contributed by Vinay Sajip in bpo-18807.)

The pyvenv command also accepts a --without-pip
option to suppress the otherwise-automatic bootstrapping of pip into
the virtual environment. (Contributed by Nick Coghlan in bpo-19552
as part of the PEP 453 implementation.)

The encoding name is now optional in the value set for the
PYTHONIOENCODING environment variable. This makes it possible to
set just the error handler, without changing the default encoding.
(Contributed by Serhiy Storchaka in bpo-18818.)

The UTF-32 decoder is now 3x to 4x faster. (Contributed by Serhiy Storchaka
in bpo-14625.)

The cost of hash collisions for sets is now reduced. Each hash table
probe now checks a series of consecutive, adjacent key/hash pairs before
continuing to make random probes through the hash table. This exploits
cache locality to make collision resolution less expensive.
The collision resolution scheme can be described as a hybrid of linear
probing and open addressing. The number of additional linear probes
defaults to nine. This can be changed at compile-time by defining
LINEAR_PROBES to be any value. Set LINEAR_PROBES=0 to turn-off
linear probing entirely. (Contributed by Raymond Hettinger in
bpo-18771.)

The interpreter starts about 30% faster. A couple of measures lead to the
speedup. The interpreter loads fewer modules on startup, e.g. the re,
collections and locale modules and their dependencies are no
longer imported by default. The marshal module has been improved to load
compiled Python code faster. (Contributed by Antoine Pitrou, Christian
Heimes and Victor Stinner in bpo-19219, bpo-19218, bpo-19209,
bpo-19205 and bpo-9548.)

bz2.BZ2File is now as fast or faster than the Python2 version for
most cases. lzma.LZMAFile has also been optimized. (Contributed by
Serhiy Storchaka and Nadeem Vawda in bpo-16034.)

By taking advantage of the new storage format for strings, pickling of
strings is now significantly faster. (Contributed by Victor Stinner and
Antoine Pitrou in bpo-15596.)

A performance issue in io.FileIO.readall() has been solved. This
particularly affects Windows, and significantly speeds up the case of piping
significant amounts of data through subprocess. (Contributed
by Richard Oudkerk in bpo-15758.)

This section covers various APIs and other features that have been deprecated
in Python 3.4, and will be removed in Python 3.5 or later. In most (but not
all) cases, using the deprecated APIs will produce a DeprecationWarning
when the interpreter is run with deprecation warnings enabled (for example, by
using -Wd).

The sysconfig key SO is deprecated, it has been replaced by
EXT_SUFFIX.

The U mode accepted by various open functions is deprecated.
In Python3 it does not do anything useful, and should be replaced by
appropriate uses of io.TextIOWrapper (if needed) and its newline
argument.

object.__format__() no longer accepts non-empty format strings, it now
raises a TypeError instead. Using a non-empty string has been
deprecated since Python 3.2. This change has been made to prevent a
situation where previously working (but incorrect) code would start failing
if an object gained a __format__ method, which means that your code may now
raise a TypeError if you are using an 's' format code with objects
that do not have a __format__ method that handles it. See bpo-7994 for
background.

difflib.SequenceMatcher.isbjunk() and
difflib.SequenceMatcher.isbpopular() were deprecated in 3.2, and have
now been removed: use xinsm.bjunk and
xinsm.bpopular, where sm is a SequenceMatcher object
(bpo-13248).

The unused and undocumented internal Scanner class has been removed from
the pydoc module.

The private and effectively unused _gestalt module has been removed,
along with the private platform functions _mac_ver_lookup,
_mac_ver_gstalt, and _bcd2str, which would only have ever been called
on badly broken OSX systems (see bpo-18393).

The hardcoded copies of certain stat constants that were included in
the tarfile module namespace have been removed.

In a posix shell, setting the PATH environment variable to
an empty value is equivalent to not setting it at all. However, setting
PYTHONPATH to an empty value was not equivalent to not setting it
at all: setting PYTHONPATH to an empty value was equivalent to
setting it to ., which leads to confusion when reasoning by analogy to
how PATH works. The behavior now conforms to the posix convention
for PATH.

The [X refs, Y blocks] output of a debug (--with-pydebug) build of the
CPython interpreter is now off by default. It can be re-enabled using the
-Xshowrefcount option. (Contributed by Ezio Melotti in bpo-17323.)

The python command and most stdlib scripts (as well as argparse) now
output --version information to stdout instead of stderr (for
issue list see Other Improvements above).

The ABCs defined in importlib.abc now either raise the appropriate
exception or return a default value instead of raising
NotImplementedError blindly. This will only affect code calling
super() and falling through all the way to the ABCs. For compatibility,
catch both NotImplementedError or the appropriate exception as needed.

The module type now initializes the __package__ and __loader__
attributes to None by default. To determine if these attributes were set
in a backwards-compatible fashion, use e.g.
getattr(module,'__loader__',None)isnotNone. (bpo-17115.)

importlib.util.module_for_loader() now sets __loader__ and
__package__ unconditionally to properly support reloading. If this is not
desired then you will need to set these attributes manually. You can use
importlib.util.module_to_load() for module management.

Import now resets relevant attributes (e.g. __name__, __loader__,
__package__, __file__, __cached__) unconditionally when reloading.
Note that this restores a pre-3.3 behavior in that it means a module is
re-found when re-loaded (bpo-19413).

Frozen packages no longer set __path__ to a list containing the package
name, they now set it to an empty list. The previous behavior could cause
the import system to do the wrong thing on submodule imports if there was
also a directory with the same name as the frozen package. The correct way
to determine if a module is a package or not is to use hasattr(module,'__path__') (bpo-18065).

Frozen modules no longer define a __file__ attribute. It’s semantically
incorrect for frozen modules to set the attribute as they are not loaded from
any explicit location. If you must know that a module comes from frozen code
then you can see if the module’s __spec__.location is set to 'frozen',
check if the loader is a subclass of
importlib.machinery.FrozenImporter,
or if Python 2 compatibility is necessary you can use imp.is_frozen().

py_compile.compile() now raises FileExistsError if the file path
it would write to is a symlink or a non-regular file. This is to act as a
warning that import will overwrite those files with a regular file regardless
of what type of file path they were originally.

importlib.abc.SourceLoader.get_source() no longer raises
ImportError when the source code being loaded triggers a
SyntaxError or UnicodeDecodeError. As ImportError is
meant to be raised only when source code cannot be found but it should, it was
felt to be over-reaching/overloading of that meaning when the source code is
found but improperly structured. If you were catching ImportError before and
wish to continue to ignore syntax or decoding issues, catch all three
exceptions now.

functools.update_wrapper() and functools.wraps() now correctly
set the __wrapped__ attribute to the function being wrapped, even if
that function also had its __wrapped__ attribute set. This means
__wrapped__ attributes now correctly link a stack of decorated
functions rather than every __wrapped__ attribute in the chain
referring to the innermost function. Introspection libraries that
assumed the previous behaviour was intentional can use
inspect.unwrap() to access the first function in the chain that has
no __wrapped__ attribute.

inspect.getfullargspec() has been reimplemented on top of
inspect.signature() and hence handles a much wider variety of callable
objects than it did in the past. It is expected that additional builtin and
extension module callables will gain signature metadata over the course of
the Python 3.4 series. Code that assumes that
inspect.getfullargspec() will fail on non-Python callables may need
to be adjusted accordingly.

importlib.machinery.PathFinder now passes on the current working
directory to objects in sys.path_hooks for the empty string. This
results in sys.path_importer_cache never containing '', thus
iterating through sys.path_importer_cache based on sys.path
will not find all keys. A module’s __file__ when imported in the current
working directory will also now have an absolute path, including when using
-m with the interpreter (except for __main__.__file__ when a script
has been executed directly using a relative path) (Contributed by Brett
Cannon in bpo-18416). is specified on the command-line)
(bpo-18416).

The removal of the strict argument to HTTPConnection
and HTTPSConnection changes the meaning of the
remaining arguments if you are specifying them positionally rather than by
keyword. If you’ve been paying attention to deprecation warnings your code
should already be specifying any additional arguments via keywords.

Strings between from__future__import... statements now always raise
a SyntaxError. Previously if there was no leading docstring, an
interstitial string would sometimes be ignored. This brings CPython into
compliance with the language spec; Jython and PyPy already were.
(bpo-17434).

The file attribute is now automatically closed when
the creating cgi.FieldStorage instance is garbage collected. If you
were pulling the file object out separately from the cgi.FieldStorage
instance and not keeping the instance alive, then you should either store the
entire cgi.FieldStorage instance or read the contents of the file
before the cgi.FieldStorage instance is garbage collected.

The complex constructor, unlike the cmath functions, was
incorrectly accepting float values if an object’s __complex__
special method returned one. This now raises a TypeError.
(bpo-16290.)

The int constructor in 3.2 and 3.3 erroneously accepts float
values for the base parameter. It is unlikely anyone was doing this, but
if so, it will now raise a TypeError (bpo-16772).

Defaults for keyword-only arguments are now evaluated after defaults for
regular keyword arguments, instead of before. Hopefully no one wrote any
code that depends on the previous buggy behavior (bpo-16967).

Stale thread states are now cleared after fork(). This may cause
some system resources to be released that previously were incorrectly kept
perpetually alive (for example, database connections kept in thread-local
storage). (bpo-17094.)

Parameter names in __annotations__ dicts are now mangled properly,
similarly to __kwdefaults__. (Contributed by Yury Selivanov in
bpo-20625.)

hashlib.hash.name now always returns the identifier in lower case.
Previously some builtin hashes had uppercase names, but now that it is a
formal public interface the naming has been made consistent (bpo-18532).

Because unittest.TestSuite now drops references to tests after they
are run, test harnesses that re-use a TestSuite to re-run
a set of tests may fail. Test suites should not be re-used in this fashion
since it means state is retained between test runs, breaking the test
isolation that unittest is designed to provide. However, if the lack
of isolation is considered acceptable, the old behavior can be restored by
creating a TestSuite subclass that defines a
_removeTestAtIndex method that does nothing (see
TestSuite.__iter__()) (bpo-11798).

unittest now uses argparse for command line parsing. There are
certain invalid command forms that used to work that are no longer allowed;
in theory this should not cause backward compatibility issues since the
disallowed command forms didn’t make any sense and are unlikely to be in use.

The re.split(), re.findall(), and re.sub() functions, and
the group() and groups() methods of
match objects now always return a bytes object when the string
to be matched is a bytes-like object. Previously the return type
matched the input type, so if your code was depending on the return value
being, say, a bytearray, you will need to change your code.

audioop functions now raise an error immediately if passed string
input, instead of failing randomly later on (bpo-16685).

The new convert_charrefs argument to HTMLParser
currently defaults to False for backward compatibility, but will
eventually be changed to default to True. It is recommended that you add
this keyword, with the appropriate value, to any
HTMLParser calls in your code (bpo-13633).

Since the digestmod argument to the hmac.new() function will in the
future have no default, all calls to hmac.new() should be changed to
explicitly specify a digestmod (bpo-17276).

Any calls to open functions that specify U should be modified.
U is ineffective in Python3 and will eventually raise an error if used.
Depending on the function, the equivalent of its old Python2 behavior can be
achieved using either a newline argument, or if necessary by wrapping the
stream in TextIOWrapper to use its newline argument
(bpo-15204).

If you use pyvenv in a script and desire that pip
not be installed, you must add --without-pip to your command
invocation.

The default behavior of json.dump() and json.dumps() when
an indent is specified has changed: it no longer produces trailing
spaces after the item separating commas at the ends of lines. This
will matter only if you have tests that are doing white-space-sensitive
comparisons of such output (bpo-16333).

doctest now looks for doctests in extension module __doc__
strings, so if your doctest test discovery includes extension modules that
have things that look like doctests in them you may see test failures you’ve
never seen before when running your tests (bpo-3158).

The collections.abc module has been slightly refactored as
part of the Python startup improvements. As a consequence of this, it is no
longer the case that importing collections automatically imports
collections.abc. If your program depended on the (undocumented)
implicit import, you will need to add an explicit importcollections.abc
(bpo-20784).

PyEval_EvalFrameEx(), PyObject_Repr(), and
PyObject_Str(), along with some other internal C APIs, now include
a debugging assertion that ensures they are not used in situations where
they may silently discard a currently active exception. In cases where
discarding the active exception is expected and desired (for example,
because it has already been saved locally with PyErr_Fetch() or
is being deliberately replaced with a different exception), an explicit
PyErr_Clear() call will be needed to avoid triggering the
assertion when invoking these operations (directly or indirectly) and
running against a version of Python that is compiled with assertions
enabled.

http.client and modules which use it, such as urllib.request and
xmlrpc.client, will now verify that the server presents a certificate
which is signed by a CA in the platform trust store and whose hostname matches
the hostname being requested by default, significantly improving security for
many applications.

For applications which require the old previous behavior, they can pass an
alternate context:

importurllib.requestimportssl# This disables all verificationcontext=ssl._create_unverified_context()# This allows using a specific certificate for the host, which doesn't need# to be in the trust storecontext=ssl.create_default_context(cafile="/path/to/file.crt")urllib.request.urlopen("https://invalid-cert",context=context)